This invention is in the field of shaped charges for use in perforating gun systems.
Installation of an oil or gas well involves fixing a tubular steel casing in cement in an underground bore. Holes are subsequently created in the steel casing and cement in order to gain access to the surrounding formation, i.e., oil or gas deposit. Such holes are generally created through a process known as perforation using a perforating gun. A well may also need to be re-perforated from time to time, for example, if the flow of oil or gas into the well becomes impeded by debris. It is also common practice to re-perforate a depleted well which is to be abandoned. Such wells are often sealed by introduction of cement between the casing and well bore to minimize leakage of remaining oil or gas. Re-perforation is carried out to open existing channels in order to improve infusion of the cement. Such specialized perforating guns are sometimes referred to in the industry as xe2x80x9cchannel finder gunsxe2x80x9d.
In general, a perforating gun includes an elongate member having several explosive charges spaced along the member. There is a detonation cord running between and connected to the charges. The charges are generally arranged along the length of the gun to explode radially outwardly in different directions into the formation. Such a charge, known as a xe2x80x9cshaped chargexe2x80x9d, includes a case, often conically shaped, which contains explosive material sealed in the case interior by a liner. When detonated, the charge delivers explosive forces to penetrate into the formation.
Efforts continue to be made to improve various aspects of the perforation process. For example, Renfro describes in U.S. Pat. No. 5,509,356 (Apr. 23, 1996), the specification of which is incorporated herein by reference, a shaped charge with an improved liner. The charge includes a housing made of a pulverable material such as a ceramic. The shaped charge includes explosive powder and a binder and the powder is pre-compressed to at least 95% of its total maximum density. The liner is generally hemispherical, but decreases in thickness from the central area to its perimeter, or skirt.
Hasselman describes in U.S. Pat No. 5,522,319 (Jun. 4, 1996), the specification of which is incorporated herein by reference, another effort to provide an improved shaped charge for oil well perforation in which a generally hemispherically shaped liner is thicker at its central region than towards its edge. This apparently improves the coherence, stability and mass distribution of the jet produced such that a tip velocity comparable to that of a conical shaped charge is obtained. The case housing of the charge is not described in detail, but is illustrated to be of uniform thickness and, being of thickness of the same order of the liner, is presumably metal. Hasselman states that the case material and configuration are of little importance to the overall design optimization, although they do have an effect and need to be included in any detailed design.
Willis et al. describe in U.S. Pat No. 5,564,499 (Oct. 15, 1996), the specification of which is incorporated herein by reference, a method and device for slotting well casing and scoring surrounding rock to facilitate hydraulic fractures. The device shown includes linear charges arranged lengthwise in the gun and concentrically about the center axis of the gun. This arrangement is described as creating a linear aperture in the well casing in which the charges are exploded.
Conventional perforating guns, however, have been found not to be entirely satisfactory when used to perforate a well in preparation for introduction of cementitious material to seal the casing to the well bore, i.e., when used as channel finder guns. The degree of perforation by such guns is sometimes insufficient to adequately open channels which can lead to insufficient infusion of the material into areas surrounding the casing. This can lead to an inadequate seal which permits leakage of formation contents, typically water, gas, or oil from between the casing and the formation. The invention disclosed herein has been found to be particularly useful in the context of this situation, although its application is not limited to such.
The present invention involves a shaped charge assembly which produces a superior performance when used as part of a perforating gun to be used as a channel finder gun. The improved performance stems from the use of a shaped charge assembly in which the charge is configured to produce a jet which is substantially non-circular in cross section (in relation to the axial direction of travel of the jet) in which the assembly includes a base that is relatively rigid with respect to the liner of the assembly.
According to a preferred embodiment, the shaped charge assembly includes a linear shaped charge.
Thus, in one broad aspect, the invention is a shaped charge assembly having: a relatively rigid base that has a cavity; a relatively flexible liner; and a shaped charge located in the cavity, wherein the charge is in a shape and the liner located such that, upon explosion of the charge, the liner is propelled along an axial direction of thrust in the formation of a jet having a cross sectional shape which is non-circular.
The cavity of the base can have an opening at a top end of the base and preferably at least a portion of the liner is located in the opening of the base and the charge is shaped such that, upon explosion thereof, the jet travels along a major axis through the opening of the base.
Preferably, the shaped charge is a linear charge.
The base can be of milled steel, stamped metal, or cast metal, etc.
The exposed, or outer surface part of the liner can be concave.
The liner (or the portion of the liner in the opening of the base) can be xe2x80x9cVxe2x80x9d-shaped and inwardly concave with respect to the top end of the base, or the liner can be xe2x80x9cWxe2x80x9d-shaped and be generally inwardly directed with respect to the top end of the base.
Particularly, interior surfaces of the base can define a longitudinal trough extending between first and second ends of the base, there can be first and second walls extending between the first and second ends, respectively, third and fourth walls extending between the first ends of the first and second walls, and the second ends of the first and second walls, respectively, and the base can include a pair of apertures therethrough for contacting detonator wires with the charge.
In a particular embodiment, illustrated below, the floor of the cavity has a central ridge extending upwardly toward the liner.
In certain embodiments, there are substantially upright interior side walls extending between the first and second ends of the base.
It is possible for the interior floor surface of the base to be generally concave upward.
In particular embodiments, interior surfaces of the base define a longitudinal trough extending between first and second ends of the base and the liner is concave inward with respect to the cavity such that the shaped material within the cavity has a substantially constant transverse cross section between the first and second ends.
There can be metal foil at the respective first and second ends, to seal the charge in the assembly.
Usually, the base and liner are each of metal.
Usually, the base has a floor having a thickness at least twice that of the liner. The base can have a floor having a thickness at least three times the thickness of the liner. The base can have a floor having a thickness at least four times the thickness of the liner. The base can have a floor having a thickness at least five times the thickness of the liner. The base can have a floor having a thickness at least six times the thickness of the liner.
Preferably, the base has walls having interior surfaces which define the cavity and the walls are relatively thick with respect to the thickness of the liner.
The explosive charge material can be compressed directly against inner surfaces of the base. The liner and base can be formed separate from each other.
The charge can be pressed into the cavity by forced abutment with the liner.
The base can be of unitary construction.
In certain embodiments, the base has an interior floor and walls having interior surfaces which extend upwardly of the floor in which the average thickness of the floor and walls is between about 2 and 10 that of the liner; or the average thickness of the floor and walls is between about 3 and 8 that of the liner; or the average thickness of the floor and walls is between about 4 and 6 that of the liner; or the average thickness of the floor and walls is between about 5 times that of the liner.
The thickness of the liner can be between about {fraction (1/32)}xe2x80x3 and xc2xcxe2x80x3. More preferably, the thickness of the liner is about {fraction (1/16)}xe2x80x3.
In particular embodiments, the invention is a shaped charge assembly in which the base is of metal and includes a floor and first and second upstanding walls at first and second opposite sides of the base, respectively, to define an upwardly open the cavity and the liner extends between the first and second walls, and the charge is compressed in the cavity such that the shape of the charge is defined by interior surfaces of the floor, walls and liner. Further, there can be first and second means for sealing the charge within the cavity, extending between first ends of the first and second walls, the liner, and the base floor, and between second ends of the first and second walls, the liner and the base floor, respectively. Such means for sealing the charge within the cavity can be a piece of foil.
Usually, interior surfaces of the first and second walls face each other and are symmetrically shaped with respect to a center line of the assembly. The interior floor surface can define an upwardly concave trough which is symmetrical with respect to the center line. In a preferred embodiment, interior surfaces of the floor, first and second walls and liner define the shape of the charge such that the charge has a generally xe2x80x9cVxe2x80x9d-shaped cross section. The cross section of the charge can be relatively constant and of a defined thickness.
In particular embodiments, the interior surface of the floor has an upwardly protruding ridge which is generally symmetrical with respect to the center line. The liner can be shaped and oriented so as to be parallel to the center line and be concave upward. In certain preferred embodiments, the liner has a generally xe2x80x9cVxe2x80x9d-shaped cross section. Alternatively, the liner can be generally xe2x80x9cWxe2x80x9d-shaped in cross section.
Typically, the inner surfaces of the first and second walls are generally parallel to the center line. Additionally, the inner surfaces of the first and second walls can be generally parallel to each other. In a preferred embodiment, the shaped charge is relatively shallow near the center of the cavity and relatively deep near the first and second walls of the base.
In particular embodiments, a shaped charge assembly of the invention includes a base of metal. The base includes a floor and first and second upstanding walls at first and second opposite sides of the base, respectively, to define an upwardly open cavity and the shaped charge is defined within a relatively flexible sheath, secured within the cavity. The sheath can be held within the cavity by frictional forces between abutting interior surfaces of the base and exterior surfaces of the sheath. The shaped charge can be oriented to explode in a direction away from the floor of the base.
Preferably, there is means for sealing exposed ends of the charge against the environment. The means can be foil. Typically, the foil is a thin metal sheet, such as aluminum foil, and includes a pressure sensitive adhesive for adherence to the charge and base surfaces. Again, the shaped charge can be a linear shaped charge. The shaped charge can have a generally xe2x80x9cVxe2x80x9d-shaped cross section, which is relatively constant from end to end of the charge. The shaped charge can have a generally xe2x80x9cWxe2x80x9d-shaped cross section which is relatively constant from end to end of the charge.
In particular embodiments, a shaped charge assembly of the invention includes a base that is of metal and includes a floor and first and second upstanding walls at first and second opposite sides of the base, respectively, and third and fourth upstanding side walls at third and fourth opposite ends of the base, respectively, to define an upwardly open cavity and the liner is secured and extends between the first, second, third and fourth walls so as to seal the charge within the cavity. Preferably, the base is of unitary construction, i.e., is formed of a single piece of material such as cast metal, for example.
In such an embodiment, it is possible to have an arrangement in which interior surfaces of the floor, first, second, third, and fourth walls, and the liner define the shape of the charge. The interior surfaces of the first and second walls can be spaced from each other a distance greater than the distance between the interior surfaces of the third and fourth walls. The interior surfaces of the third and fourth walls can slope upwardly and outwardly. The third and fourth walls preferably are symmetrically shaped with respect to a center line of the assembly.
It is possible for interior surfaces of the walls to be generally upright with respect to the interior surface of the floor. In certain illustrated embodiments, there is a ridge extending upwardly of the floor running parallel to a center line of the assembly running between the first and second walls.
The liner can be generally concave inward. The liner can be generally parallel to a center line of the assembly running between the first and second walls. The liner can have a generally xe2x80x9cVxe2x80x9d-shaped cross section. The liner can have a generally xe2x80x9cWxe2x80x9d-shaped cross section.
Preferably, the charge is compressed directly against the inner surfaces and the liner and base are formed separate from each other.
The base can include a pair of apertures therethrough for contacting detonator wires with the charge located in the cavity of the base.
In particular embodiments, a shaped charge assembly of the invention has a base that is of metal and includes a floor and first and second upstanding walls at first and second opposite sides of the base, respectively, and third and fourth upstanding side walls at third and fourth opposite ends of the base, respectively, to define an upwardly open cavity and the shaped charge is defined within a relatively flexible sheath, secured within the cavity.
In such embodiments, it is preferred that the sheath is held within the cavity by frictional forces between abutting interior surfaces of the base and exterior surfaces of the sheath. Often, the shaped charge is oriented to explode in a direction away from the floor of the base. The shaped charge can be a linear shaped charge. The shaped charge can have a generally xe2x80x9cVxe2x80x9d-shaped cross section, and can be of relatively constant thickness from end to end of the charge. The shaped charge can have a generally xe2x80x9cWxe2x80x9d-shaped cross section which is relatively constant from end to end of the charge.
In a particular embodiment, the invention is a shaped charge assembly for mounting to a charge holder of an underground perforating gun including: a rigid base which defines a cavity open at a top end of the base; charge material received within the cavity; and a liner, relatively flexible with respect to the base, mounted at the top end of the cavity to seal the material within the cavity; and wherein, interior surfaces of the base and the liner together define the shape of the charge material, which shape is such that, upon detonation, a jet with a non-circular cross-section travels along a major axis through the open end of the cavity of the base.
In another particular aspect, the invention is a combination of a shaped charge assembly and a holder therefore, for use in a longitudinal housing of a perforating gun. The holder defines a slot for receipt of the assembly therein to orient the shaped charge assembly within the housing such that, upon explosion of the charge, the axial direction of thrust is in a direction generally orthogonal to a longitudinal axis of the housing.
The holder usually defines a plurality of slots. Often, each respective shaped charge assembly received in a slot is oriented such that, upon explosion of the charge, the jet has a major planar component through a central thrust axis of the jet, which planar component is non-parallel with the longitudinal axis of the housing. In a particular illustrated embodiment, the planar component is generally orthogonal to the longitudinal axis of the housing.
In such a combination in which there is a plurality of shaped charge assemblies, neighboring shaped charge assemblies can be oriented with respect to each other such that their central thrust axes form an angle of between 10xc2x0 and 90xc2x0 with each other, or an angle of between about 20xc2x0 and 70xc2x0 with each other, or an angle of between about 30xc2x0 and 60xc2x0 with each other, or an angle of about 45xc2x0 with each other. It possible for assemblies to be dimensioned to permit a density of up to about 16 shaped charge assemblies per meter in a holder, or a density of up to about 14 shaped charge assemblies per meter in a holder, or a density of up to about 12 shaped charge assemblies per meter in a holder, or a density of up to about 10 shaped charge assemblies per meter in a holder.
Such a combination can further include a gun housing.
In another aspect, the invention is a method for perforating a casing of a well bore and a surrounding hydrocarbon-bearing formation. The method includes:
locating at least one shaped charge assembly in the bore, the assembly having a relatively rigid base, a shaped charge in a cavity of the base, and a relatively flexible liner; and
exploding the charge, wherein the charge is shaped and oriented within the bore, to produce upon explosion thereof, a jet having an axial direction of thrust in a plane generally orthogonal to the bore and wherein the cross sectional shape of the jet is non-circular.
In another aspect, the present invention is a method for perforating a casing of a bore of a well and a surrounding hydrocarbon-bearing formation which includes:
locating at least one shaped charge assembly of the invention in the bore; and
exploding the charge.
The shaped charge can include:
a relatively rigid base having a cavity;
a relatively flexible liner; in which there is:
a shaped charge located in the cavity, wherein the charge is in a shape and the liner located such that, upon explosion of the charge, the liner is propelled along an axial direction of thrust, located in a plane generally orthogonal to the bore, in the formation of a jet having a cross sectional shape which is non-circular.
A particular method is one in which the perforating is conducted in preparation for sealing the well.